One example of an optical communication system is a passive optical network (PON) system. In order to increase the transmission capacity of the PON system, the signal transmission speeds have been increased. However, rather than on serial high-speed transmission, studies have been conducted on configurations in which the total amount of data to be transmitted through one optical fiber is increased by using different wavelength channels without increasing the speed of the signals, i.e., studies have been conducted on PON systems that use wavelength division multiplexing (WDM) (hereinafter, wavelength division multiplexing PON systems). In a wavelength division multiplexing PON system, a plurality of usable wavelengths are allocated to each of the communication (downstream communication) in the downstream direction, i.e., from the optical line terminal (OLT) toward the optical network units (ONUs), and the upstream communication, which is in the opposite direction. Each ONU communicates with the OLT by using one of the usable wavelengths (for example, see Patent Literature 1).
For example, when 32 ONUs (=32 users) are accommodated, in a PON system that does not perform wavelength division multiplexing, the transmission band for one wavelength is shared by 32 users. In contrast, in a wavelength division multiplexing PON system that is configured to use four waves for each of the upstream wavelength and the downstream wavelength, the number of users to be accommodated per wavelength can be reduced to eight (when the users are equally allocated to each wavelength). In other words, the band to be allocated to one user can be quadrupled.